Thin film transistors show various properties depending on materials used in a semiconductor layer therein. For example, if a low temperature polysilicon semiconductor is used as the semiconductor layer, a thin film transistor of good reliability can be achieved. Furthermore, if a oxide semiconductor is used as the semiconductor layer, a thin film transistor of a small off current can be achieved. If a thin film transistor including a polysilicon semiconductor layer and a thin film transistor including an oxide semiconductor layer are formed on the same substrate, the oxide semiconductor layer may become low resistive because of hydrogen diffusion from the polysilicon semiconductor layer to the oxide semiconductor layer, and a desired property may not be obtained.